The invention relates generally to an optic fiber sensing cable, and more particularly to a hermetically sealed fiber sensing cable.
Fiber Bragg gratings can be wavelength multiplexed along one fiber, making them attractive for measurements of strain and temperature. The fiber Bragg gratings can also be used as a pressure-temperature sensor by measuring the shift in Bragg wavelength caused by a change in hydrostatic pressure or a change in temperature, thereby providing a simple sensor design with small dimensions and good reproducibility and long-term stability. Fiber optic sensors are widely used in the oil and gas industry for monitoring down-hole parameters such as pressure, temperature, hydrocarbon flow and seismic status. However, when the fiber Bragg grating sensors are operated under conditions of high temperature, such as in oil and gas wells, a considerable drift effect both in fiber Bragg grating and birefringent interferometric sensors may be observed. The drift effect is believed to occur when the fiber is surrounded by a fluid, such as water, oil, or gaseous hydrocarbons. This drift effect is found to increase with increase in temperature.
The drift effect may be due to ingress of a liquid such as water into the fiber cladding resulting in the development of additional tensile stress on the core fiber. Generally, such stress may modify the effective refractive indices in both the fiber cladding and the fiber core resulting in changes in the resonant wavelength of a Bragg grating disposed within the core fiber. In addition, diffusion of gases, such as hydrogen, into the fiber, may cause a change in the refractive index proportional to the hydrogen concentration, and consequently causes a drift in the resonant wavelength. Therefore, there is a need to provide fiber sensing cables which limit the ingress of exogenous materials such as water and hydrogen into the optic fiber, thereby limiting transmission losses and increasing the reliability of the fiber sensing cable.
One solution known in the art is to apply a carbon coating, or other hermetic coating on the outer surface of the cladding surrounding the core fiber of the Bragg grating sensor in order to protect the optical fiber and sensors. Carbon has been shown to provide a good hermetic coating for optical fibers, making them less permeable to both water and hydrogen. However, one of the disadvantages of using a carbon coating is that conventional carbon coatings are thermally stable up to a temperature of only about 300 degrees centigrade. Beyond this temperature, the carbon layer begins to lose its hermeticity.
Thus, conventional carbon coatings have to be protected by an outer polymer coating such as a polyimide coating layer, in order to protect the carbon from losses resulting from high temperature processes such as oxidization. Generally, however, the protective polymer layer is itself subject to decomposition at high temperature, and the level of protection afforded by a protective polymer coating atop the carbon layer is itself limited. This generally limits the use of carbon-coated optic fiber sensing cables to temperatures less than about 300° C. As will be appreciated by those of ordinary skill in the art, temperatures in hydrocarbon wells or geothermal wells, in which the fiber optic sensing cables might otherwise be used to advantage, may exceed the operating temperature capabilities of the fiber optic sensing cables, thereby marginalizing their utility to monitor conditions within such wells.
Metal coating layers have also employed as an alternative to conventional carbon coatings. However, the use of the metal coating layers may result signal attenuation due micro-bending effects, and field energy leakage from the core fiber to the cladding.
Therefore there remains a need for hermetically sealed fiber sensing cables which resist the ingress of water and hydrogen into the fiber cladding and core fiber even when the sensing cable is used in harsh environments, for example at temperatures in excess of 300° C., in order to avoid signal transmission losses while retaining the adequate mechanical strength.